Comment by scrumbledober

I don't know what the actual website is, but the Dungeon Master for my DnD group sent out a link to a collaborative calendaring website and all we needed was the link, and we just put our names in and marked out our availability. This really only worked because we didn't have to sign in or create accounts, just have the link. There are probably other low-impact ways to handle the authentication, I would probably just use a sign in with google page.